1. Field of the Invention
The present invention relates to a semiconductor device, such as a semiconductor memory device, for employing a ROM circuit to adjust the operational characteristics of a refresh counter and a delay circuit, for example, when they differ as a result of a manufacturing variance.
2. Related Arts
In a dynamic random access memory (DRAM), one type of semiconductor memory device, storage of data depends on whether electric charges are accumulated in memory cells or not in accordance with stored data. A memory cell in DRAM, which is most commonly employed, is constituted by one transistor and one capacitor. Electric charges are accumulated in the capacitor and the transistor is selected by a word line and a bit line.
Electric charges can be held in a DRAM memory cell for a predetermined period of time, and stored data can be precisely read therefrom. However, since the accumulated electric charge leaks from the capacitor of the memory cell, an inadequate electric charge is retained by the capacitor after the predetermined period of time has elapsed so that the stored data can not be precisely read. Thus, for DRAM, refresh operations are performed during which memory cells are accessed in a predetermined cycle and stored data are rewritten in the memory cells.
Recently, in order to reduce system load, a self-refresh function has been incorporated in DRAM, and data in memory cells are rewritten in the memory cell at a predetermined cycle by a refresh counter constituted by an internally provided oscillator.
With respect to another things, a clock signal delayed a predetermined time is generated by a delay circuit in a semiconductor circuit. The semiconductor circuit, for example, has a functional circuit determining whether an external clock signal is detected between an internal clock signal and the delayed clock signal.
It is inevitable that the above refresh counter and the delay circuit will be accompanied by manufacturing variances. Therefore, a programmable read only memory (PROM) for adjusting characteristic values is internally provided so that the manufacturing variances can be absorbed and characteristic values can be adjusted.
However, a great deal of time and labor is required to adjust characteristic values for using a PROM. In other words, an adjustment for a characteristic value and confirmation of a circuit operation must be repeated. The adjustment of a characteristic value is performed, for example, by employing an adjustment signal stored in the PROM to adjust the characteristic values affected by manufacturing variances. Confirmation of an operation is performed by examining an internal circuit thus adjusted to determine whether it operates correctly. When correct operation is not confirmed, another adjustment must be performed by employing the PROM circuit.
There are some adjustment PROMS, such as fuse ROMS, wherein data once written can not be corrected. If such a PROM is used for adjustment, it can not be programmed for re-adjustment. Therefore, when correct operation can not be confirmed following a first adjustment, a product will have a defect which can not be corrected.